Various types of apparatuses use a gear driving system that drives a driving target by transmitting a generated driving force of a driving source to the driving target via a driving transmitting member, or more specifically, gears. One of the key factors in the apparatuses using the gear driving system is designing of the gear driving system.
Conventionally, designing of the gear driving system has primarily focused on obtaining gear dimensions and gear materials required to obtain desired characteristics, such as the reduction ratio and torque output, within an available space.
However, the gear designing system so designed, despite a proper reduction ratio or output torque, may cause oscillation in the final gear of the gear driving system when the system is installed in an apparatus for actual applications. More specifically, depending on combinations of the gears, the driving force that rotates the driven gear may not smoothly transmit to the driven gear, with the result that oscillation is caused. Such oscillation is known as, for example, a banding phenomenon.
For example, in an application where the gear driving system is used in a driving section of an image forming apparatus of an electrophotographic type to rotate a photoreceptor roller, the banding phenomenon in the gear driving system causes non-uniformity in the peripheral velocity of the photoreceptor roller, causing unevenness in charging, exposure, and development. Such “pitch unevenness” is also caused when the banding phenomenon occurs in a gear driving system used in a sheet transport system.
In other applications, the banding phenomenon might occur, for example, in a document reading optical system to cause uneven driving. In this case, the moving speed of the optical unit becomes non-uniform, which may produce missing or duplicate part in the read-out image, failing to faithfully read the document.
Further, when the banding phenomenon causes uneven driving in, for example, a driving section of a fixing mechanism, the peripheral velocity of the fixing roller becomes non-uniform, which may cause uneven transfer of heat in the unfixed toner passing the fixing mechanism, with the result that over-fixing or fixing error is caused. Further, this may impair the glossiness of the print.
The banding phenomenon thus impairs print quality in an image forming apparatus, for example.
One example of a structure for reducing the banding phenomenon is Japanese Publication for Examined Patent Application No. 48390/1992 (Tokukouhei 4-48390; published on Aug. 6, 1992), which discloses a structure using helical gears 101 and 102 for a drum gear used as the driving system of a photoreceptor drum, as shown in FIG. 8.
The helical gears 101 and 102 used in this publication are provided to exert a thrust force in a direction of the rotational axis of a photoreceptor drum 103 to maintain the position of the photoreceptor drum 103 in a direction of the axis line. This structure can reduce the banding phenomenon in the driving system because the gear driving system using the helical gears has weaker teeth engagement than the gear driving system using spur gears.
Japanese Publication for Unexamined Patent Application No. 147948/2000 (Tokukai 2000-147948; published on May 26, 2000) discloses a photoreceptor unit for reducing jitter (oscillation) caused by teeth engagement, by using smaller modules for a photoreceptor gear 111 and a photoreceptor linking gear 112, and by using large modules for a photoreceptor linking gear 113, a driving input gear 114, and a driving output gear 115, as shown in FIG. 9.
However, the foregoing conventional structures are associated with the following problems.
Namely, in order to reduce oscillation in the gear driving system to a desired level, the structures disclosed in the foregoing publications Tokukouhei 4-48390 and Tokukai 2000-147948 both require the experience and know-how of a designer. Further, some trial-and-error is required to obtain optimum design conditions. Thus, design efficiency is low in these prior art techniques.
For example, in Tokukouhei 4-48390, while the use of the helical gears may reduce oscillation, it is up to the experience and know-how of the designer to determine characteristics of the gears as to whether, for example, how many teeth should each gear have to minimize oscillation.
Further, in Tokukouhei 4-48390, the use of the helical gears increases the driving load and thus power consumption. That is, it is difficult to optimize the overall characteristics of the driving system, including reducing power consumption.
Further, the helical gears in Tokukouhei 4-48390 and the gears with small modules in Tokukai 2000-147948 are used only for the final gear of the gear driving system. This requires the number of gears to be changed for purposes other than adjusting the reduction ratio of the gears, i.e., for the purpose of changing the type of gear in the final stage of the gear train. This is disadvantageous in realizing smaller apparatuses.